U.S. Pat. No. 4,524,236 discloses a process for the oxidative dehydrogenation (oxydehydrogenation) of ethane, in which a calcined composition of the elements Mo, V, Nb, Sb as the catalyst [sic] and X=Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, Zn, Cd, Hg, Al, Tl, Pb, As, Bi, U, W, Te is used. This catalyst provides a conversion of up to 73% at a selectivity of 69% for ethylene at 400.degree. C. and a weight-related space velocity (WHSV) of 1.5 h.sup.-1. The abovementioned patent states that the catalyst, because it does not efficiently oxydehydrogenate propane, n-butane or i-butane, but instead combusts these gases to carbon dioxide and other oxidized carbon products, is essentially limited to the oxydehydrogenation of ethane to ethylene.
The article "Selective Oxidation of Methane and Ethane of Li.sup.- --MgO-Cl.sup.- Catalysis Promoted with Metal Oxides" by S. J. Conway, D. J. Wang and J. H. Lunsford in Applied Catalysis A79, pp. L1 to L5 (1991) discloses catalysts for the oxydehydrogenation of ethane to ethylene that include magnesium oxide and lithium oxide, chlorine and other metals from the La, Nd and Dy group. This catalyst [sic] provides a conversion rate of 83.8% for ethane with a selectivity for ethylene of 63.8% at a temperature of 585.degree. C. and a WHSV of 0.18 h.sup.-1. No further information is available about the performance of this catalyst in the oxydehydrogenation of LPG components.
U.S. Pat. No. 1,777,319 discloses a vanadium-based catalyst for the oxydehydrogenation of C.sub.2 to C.sub.8 paraffins at temperatures of 300 to 700.degree. C. The formula of the catalyst is M.sub.3 (VO.sub.4).sub.2 and/or MV.sub.2 O.sub.6, where M is one of the elements Mg, Zn, Ca, Pb or Cd. The inventors of this catalyst disclose detailed information in later articles.
In these articles, "Selective Oxidative Dehydrogenation of Butane over V--MG--O Catalysts" by M. A. Chaar, D. Partel, M. C. Kung and H. H. Kung journal of Catalysis 105, pp. 483-498, 1987), "Selective Oxidative Dehydrogenation of Propane over V--MG--O Catalysts" by M. A. Chaar, D. Partel and H. H. Kung journal of Catalysis 109, pp. 463 to 467, 1988) and "Selectivity Patterns in Alkane Oxidation over Mg.sub.3 (VO.sub.4).sub.2 to MgO, Mg.sub.2 V.sub.2 O.sub.7 and (VO).sub.2 P.sub.2 O.sub.7 " by P. M. Michalakos, M. C. Kung, I. Jahan and H. H. Kung journal of Catalysis 140, pp. 226 to 242, 1993), the authors disclose mixed oxide catalysts that contain vanadium and magnesium. The catalysts are active at temperatures in the range of 475 to 540.degree. C. for the oxydehydrogenation of propane, butane and isobutane. At 540.degree. C. and WHSV=2 h.sup.-1, conversion is achieved for butane at a level of up to 58.9% and for propane of 35.8%. At 500.degree. C. and WHSV=6.5 h.sup.-1, conversion of 12% is achieved for isobutane with a selectivity for corresponding olefins and butadiene of 48.8% and 42.4% and 53.0%. No information is available about the conversion of LPG over V--MgO catalysts. Based on the data for the pure components, it can be concluded that for LPG containing roughly 50% propane, a conversion of 40% and a selectivity of 50% can be achieved with an olefin yield of roughly 20% per passage at WHSV=2 h.sup.-1. This was confirmed by tests carried out in the framework of studies for the present invention.
An important feature of these V--Mg catalysts is the high selectivity of the oxydehydrogenation path. No cracking reactions were observed. Another feature of these catalysts is that the main product of the dehydrogenation of n-butane is butadiene, whereby the selectivity for C.sub.4 H.sub.6 is 37.7%, with a total selectivity of 48.8%, and the yield of butadiene, among all dehydrogenation products, is 77.2%.
The article by D. Bhattacharyya, S. K. Bej and M. S. Rao, "Oxidative Dehydrogenation of n-Butane to Butadiene. Effect of Different Promoters on the Performance of Vanadium-Magnesium Oxide Catalysts" (Applied Catalysis A87, pp. 29 to 43, 1992), discloses a mixed oxide catalyst that consists of a mixture of vanadium, magnesium and a third component made from Mo, Cr and Ti or Cr and Ti. At 570.degree. C. and WHSV=0.8 h.sup.-1, this catalyst provides a conversion of 59% and a selectivity of 53% with a yield of oxydehydrogenation products of 33.8%, of which 70% is butadiene.
Studies for the present invention have shown that, in contrast to the catalyst known from U.S. Pat. No. 4,524,236, the Li--Mg--X--Cl catalyst known from Applied Catalysis A79, pp. L.sub.1 to L5 (1991), which was developed for the oxydehydrogenation of ethane, has a high selectivity in LPG conversion for olefins at 600.degree. C. and WHSV=0.18 h.sup.-1. An increase in WHSV for the LPG causes a drastic reduction in LPG conversion: At WHSV=1 h.sup.-1 and 600.degree. C., the conversion was 10.9%, with roughly the same selectivity of 80%.
Further features of this Li--Mg--X--Cl catalyst are the partial cracking of butane, propane and isobutane under the conditions of oxydehydrogenation, whereby corresponding low olefins and methane are obtained, and negligible amounts of butadiene in the oxydehydrogenation products.
In summary, it can be said that according to the available data, the best catalyst allows a high LPG conversion with low selectivity to be achieved for olefins at a WHSV of roughly 2 h.sup.-1 (V--Mg--O basis catalyst) or a high selectivity for olefins with a low conversion at a WHSV&gt;1 h.sup.-1 (Li--Mg--X--Cl basis catalyst).
The object of the present invention is to provide a novel catalyst and a process using this catalyst for the oxidative dehydrogenation (and cracking) of C.sub.2 to C.sub.5 paraffins to C.sub.2 to C.sub.5 olefins at high conversion rates and simultaneously with high selectivity for olefins.